Brief Description of the Prior Art
Sulfomethylation of polyacrylamide (PAM) polymers is reported by Schiller and Suen, Industrial Engineering Chemistry, Vol. 48, No. 12, pages 213237, December 1956. Those procedures describe introduction of sodium sulfomethyl groups into polyacrylamide through reaction with formaldehyde and sodium bisulfite at temperatures of 50 to 75.degree. C. at pH levels above 10. An investigation of Bakalik and Kowalski, J. Polymer Science, Vol. 25, pp. 433-436 (1987) indicated that no sulfomethylation of polyacrylamide occurs by the Schiller and Suen procedure under conditions of relatively high pH and the comparatively low temperatures specified. Particularly, it has been found that the polyacrylamide undergoes hydrolysis in the reaction milieu and that a complex equilibria species form among formaldehyde, bisulfite and the ammonia that is generated.
Scanley, U.S. Pat. No. 4,090,992 (1978) describes sulfomethylation of polyacrylamide (PAM) latex (water-in-oil) using bisulfite and formaldehyde by a procedure which requires drying of the water-in-oil (PAM) latex to reduce the water content to less than 40% (based on polymer and water) and conducting the sulfomethylation at temperatures of 50-55.degree. C.
It has more recently been determined that in the sulfomethylation of high molecular weight latex PAM's in water media, the sodium aminomethane sulfonate exists in equilibrium with formaldehyde and sodium bisulfite (NaHSO.sub.3). While not completely understood, it is believed that the presence of free formaldehyde in the reaction medium may result in cross linking of the polymer which is reflected in undesirable lower or unmeasurable RSV's.